1. Field of the Invention
This invention relates to general purpose monovinylidene aromatic polymer resins. In one aspect, this invention deals with styrenic polymer resin compositions having improved combinations of heat distortion, tensile strength, and melt flow properties. In another aspect, this invention deals with a process for the preparation of styrenic polymer resins having such improved combinations of properties. This surprising combination of properties is achieved by producing a polymer composition having a particular molecular weight distribution. This invention allows the production of strong heat-resistant molded articles while reducing or eliminating the need for plasticizing additives (typically a mineral oil). The particular molecular weight distribution can readily be achieved by blending a minor portion of a high molecular weight polymer with a major portion of an intermediate molecular weight polymer.
2. Description of the Prior Art
In general, the basic physical properties that are usually desired in the production of molded thermoplastic resin articles are an easily processable starting resin which produces articles that are relatively strong and heat resistant. It is generally known that relatively strong, heat resistant articles can be produced from monovinylidene aromatic polymer resins. It is also well known that most such resins otherwise suitable for producing articles possessing these properties must be modified in some way to improve their melt flow characteristics enough to allow them to be both easily and rapidly processed on available processing equipment, such as injection molding equipment, under normal conditions.
The prior art approaches to improving the processability of styrenic resins have involved (a) adding a plasticizing compound such as mineral oil; (b) lowering the average molecular weight of the resin; or (c) broadening the molecular weight distribution of the resin to increase the size of the lower molecular weight fraction and decrease the number average molecular weight to improve the melt flow rates.
It is well known in the thermoplastic molding art that there are several problems associated with plasticizing additives. Most plasticizers, while improving the melt flow characteristics of a thermoplastic resin, rapidly decrease its Vicat Heat Distortion temperature (VHD). Mineral oil, a very commonly used plasticizer, not only reduces the VHD of a resin, it can also cause die drip, fogging and smoking during processing.
With regard to improving processability by lowering the average molecular weight of the resin, it has been found that while reducing the average molecular weight of monovinylidene aromatic polymer resins can yield desirable melt flow characteristics and processability, there is a noticeable reduction in the strength of the articles made therefrom.
It is also known that broadening the molecular weight distribution of monovinylidene aromatic polymer resins can achieve beneficial results with regard to the processability mainly due to the increase in the amount of lower molecular weight polymer present therein. However, the increased processability achieved by broadening the molecular weight distribution of the resin is usually accompanied by problems associated with die swell occurring during processing as well as loss of strength and lowered heat distortion temperatures of articles formed therefrom. See, for examples of broadening molecular weight distributions, U.S. Pat. No. 4,137,388 to Cutter which teaches increasing the amounts of both the low and high molecular weight fractions in polystyrene resins to improve the melt flow and strength properties of polystyrene. See also, for example, Japanese Patent Announcement Kokai No. 55-164,207/1980 which similarly teaches broadening the molecular weight distribution in both the low and high molecular weight areas to achieve superior heat resistance, impact resistance and molding characteristics. See also U.S. Pat. No. 4,187,260 to Kruse, et al. where it is taught that it is desirable to increase the amount of high molecular weight monoalkenyl aromatic polymer in the matrix phase of a rubber-modified monoalkenyl aromatic polymer to balance the large amounts of low molecular weight polymer produced while grafting large amounts of polymer onto the rubber.
As is shown by the teachings of these patents, it is generally believed that it is necessary to have increased amounts of low molecular weight polymer, sometimes balanced with amounts of high molecular weight polymer, in order to achieve desirable combinations of melt flow and strength properties. The low molecular weight fractions are generally believed responsible for such improved melt flow properties. It is also generally believed that an increase in the amount of high molecular weight polymer, while improving the physical properties of molded articles, will result in decreased processability and necessitate the use of plasticizing additives such as mineral oil or low molecular weight polymer in order to achieve suitable processability.
3. Objects of the Invention
In view of what is known in the art, it would be desirable to have monovinylidene aromatic polymer compositions suitable for use as molding resins, which resins would possess melt flow properties which make them easily processable, reducing or preferably eliminating the need for plasticizing additives, which resins would facilitate the production of strong, heat resistant articles.
It would be very desirable, in view of the detrimental effects of low molecular weight polymer in a monovinylidene aromatic polymer resin, to produce resins possessing such good melt flow properties without substantially increasing the size of the low molecular weight fractions of such resins. It would be most desirable if strong, heat resistant, easily processable resins could be obtained by blending amounts of certain high molecular weight resins with intermediate molecular weight base resins. It would also be desirable if the production of these blends could be done in a continuous polymerization process.